1. Field of the Invention
The present invention generally relates to apparatus and methods for assembling pipes, and more particularly to methods and systems for joining tubes for solar receivers.
2. Discussion of the Background
Solar thermal power plants may be used to obtain electric power from the sun. In such plants, the solar flux impinges on tubes through which a heat exchange medium flows. In some solar thermal power plants, tubes are situated in a solar collector, such as along the axis of a parabolic trough. The heated heat exchange medium from the tubes may be used in a thermodynamic cycle to generate electric power.
FIG. 1A is a perspective view of a portion of a typical prior art concentrating solar power plant 100 comprising one or more solar energy collectors 110 arranged in a field. Each collector 110 includes one or more trough-shaped structures 113 having a reflective surface 119, two or more ground supports 111, an absorber tube 115 that extends the length of the collector, and tube supports 117 that couple the reflector to the absorber tube. It is not uncommon for each collector 110 to have a length A of approximately 380 feet (116 meters), a width B of approximately 20 feet (6 meters), and a height off the ground H greater than 10 feet (3 meters).
Typically, surfaces 119 have a longitudinal axis along length A and a parabolic shape in a plane perpendicular to the longitudinal axis, and absorber tube 115 is supported along the axis, such that light normally impinging on the reflector is focused (or concentrated) on the absorber tube. A mechanism (not shown) is provided to so rotate reflective surface 119 during the day to direct incident sunlight on absorber tube 115 and thus optimize the collection of solar energy on the tube.
Absorber tube 115 is generally hollow to permit the flow of a heat transfer medium, such as water, salt, or some other liquid or gas, along the absorber tube, thus collecting the concentrated solar energy. The exiting heat transfer medium may then, for example, be used to drive a turbine or heat engine (not shown) to generate electricity.
The construction of certain solar power plants 100 generally involves the following steps: 1) placing ground supports 111 in the field, 2) attaching trough-shaped structures 113 to the ground supports, and 3) joining absorber tube 115 to tube supports 117. To facilitate construction, absorber tube 115 may be formed by joining many smaller tubes that are joined together. The smaller tubes are sometimes referred to as “solar receiver tubes” or “heat collection elements (HCE).”
FIG. 2 is a partial sectional side view of a prior art HCE 200, FIG. 3 is an end view of the HCE, and FIG. 4 is a sectional end view of the HCE. HCE 200 may be, for example and without limitation, a SCHOTT solar receiver tube model PTR 70 (SCHOTT Solar, Inc., Albuquerque N. Mex.).
Typically, HCE 200 includes an outer tube 210 having a diameter D that is capped at each end by a metal flange 215, an inner tube 211 and that is coaxial with the outer tube, and a metal bellows 213 that connects the flange and inner tube. Tube 210 is preferably optically transparent and is made, for example of a glass. Flange 215 is attached to a bellows 213 that extends to tube 211. Tube 211 is thermally conductive, and may be formed from a metal, and has a length L and an inner diameter d, through which a heat transfer medium may flow. Tube 210 is generally transparent to sunlight to facilitate the solar heating of a heat exchange medium that may flow through glass tube 211, as indicated by arrows in FIG. 2. Tubes 210 and 211, bellows 213 and flange 215 are sealed to form a volume 212, which is evacuated to provide a high thermal insulation between tubes 210 and 211.
In general, the length L is from 5 feet (1.5 m) to 20 feet (6 m), the diameter D is from 2 inches (50 mm) to 7 inches (0.18 m), and the diameter d is from 1 inch (25 mm) to 4 inches (0.1 m).
For certain HCEs 200, tube 211 protrudes longitudinally beyond the end of each flange 215 by a distance S, which it typically from 0.375 inches (10 mm) to 4 inches (0.1 m). The portion of tube 211 that so protrudes is referred to as a collar 214. Forming an absorber tube 115 requires joining collars 214 of adjacent absorber tubes. In certain other HCEs 200, the free ends of flange 215 may also have a radial protrusion at the end.
FIG. 1B is a perspective view illustrating details of a prior art solar energy collector 110. Absorber tube 115 is formed from a plurality of HCEs 200, denoted 200a, 200b, and 200c. The ends of each pair of HCEs are support by one tube support 117.
One method for joining HCEs 200 is by orbital welding. One example of such a welder system is an Arc Machines model 207 power supply controller (Arc Machines, Inc., Pacoima, Calif.) with its mating 207-CW cooling package may be used with an Arc Machine 9-7500 welder.
Due their length, L, and glass components, solar receiver tubes tend to be fragile, and difficult to join, typically by welding, since the collars 214 protrude beyond the ends of the glass outer tube 210 by a relatively small distance from each end. Further, collars 214 are adjacent to bellows 213, on whose integrity the vacuum of volume 212 depends. In addition, the height C may make it very difficult to place and manipulate a welder. Solar receiver tube are thus difficult to join, especially in the field, without damaging the more fragile glass outer tube 210 or the bellows 213 joining tubes 210 and 211. There is a need in the art for methods and apparatus that permit the easy and rapid joining of such tubes to facilitate more efficient assembly of solar energy systems.